In an information recording medium, such as a CD-ROM (Compact Disc-Read Only Memory), a CD-R (Compact Disc-Recordable), a DVD-ROM, a DVD-R, a DVD-RW, and a DVD+R, for example, as described in patent documents 1 and 2 or the like, there is also developed an information recording medium, such as a multilayer type or dual layer type optical disc, in which a plurality of recording layers are laminated or pasted on the same substrate. Then, on an information recording apparatus, such as a DVD recorder, for performing the recording with respect to the dual layer type, i.e., two-layer type, optical disc, laser light for recording is focused or condensed on a recording layer located on the front (i.e. on the closer side to an optical pickup) as viewed from the irradiation side of the laser light (hereinafter referred to as an “L0 layer”, as occasion demands) to thereby record information into the L0 layer in an irreversible change recording method by heat or a rewritable method. Moreover, the laser light is focused or condensed on a recording layer located on the rear of the L0 layer (i.e. on the farther side from the optical pickup) as viewed from the irradiation side of the laser light (hereinafter referred to as an “L1 layer”, as occasion demands), through the L0 layer or the like, to thereby record information into the L1 layer.
When information is recorded into the L1 layer, which constitutes such a two-layer type optical disc, the optimum recording power of the laser light with which the L1 layer is irradiated through the recorded L0 layer, as shown in FIG. 12(a), is 44.5 (mW: milliwatt) at which a jitter value is minimal on a parabolic curve in a thin line (with white triangles) in FIG. 12(c), for example. On the other hand, the optimum recording power of the laser light with which the L1 layer is irradiated through the unrecorded L0 layer which has a different light transmittance from that of the recorded L0 layer, as shown in FIG. 12(b), is 46 (mW: milliwatt) at which a jitter value is minimal on a parabolic curve in a thick line (with black triangles) in FIG. 12(c), for example. Thus, there is a need to consider whether or not the L0 layer is recorded, in the case of the recording in the L1 layer. With respect to this, there is devised or invented a recording method in which a so-called recording order is satisfied, which is that the laser light for recording which has penetrated or transmitted the L0 layer in a recorded state is to be irradiated, for example.
The recording order is now discussed. When the information is recorded into the L1 layer, if the recording power is set to appropriately perform the recording in the case that an area in the L0 layer is in the recorded state, for example, the amplitude of a reproduction HF (High Frequency) signal is large, and appropriate or good signal quality is obtained, as shown in the left part of FIG. 13, in an area irradiated with the laser light for recording through the L0 layer in the recorded state. In other words, an asymmetry value obtained from this signal takes an appropriate value. On the other hand, as shown in the right part of FIG. 13, if the recording power is set to appropriately perform the recording in the case that the area in the L0 layer is in the recorded state, for example, the amplitude of the reproduction signal is small, and good signal quality is not obtained in an area irradiated with the laser light for recording through the L0 layer in an unrecorded state. In other words, the asymmetry value obtained from this signal takes a different value from the appropriate value, such as a low value. On the other hand, as shown in the middle part of FIG. 13, the amplitude of the reproduction signal in an area irradiated with the laser light for recording through the L0 layer in which the recorded area and the unrecorded area are mixed on a single track, varies depending on how eccentrically or disproportionately the area of the L0 layer in the recorded state or the area of the L0 layer in the unrecorded state is located from the center of the irradiation diameter of the laser light in one circle (or track), due to the extent of an eccentric amount. In other words, the asymmetry value obtained from this signal has such a feature that it transits from one to the other out of the high level and the low level. If the recording with the irradiation diameter of the laser light penetrating the L0 layer in the completely recorded state is not performed, there likely arises a problem in the reproduction.
Thus, there have been devised or invented a recording method based on the above-mentioned recording order, in such a recording condition that the recording edge of the L1 layer is set a laser irradiation radius or more away from the recording edge of the L0 layer in order to record with the irradiation diameter of the laser light penetrating the L0 layer in the completely recorded state. Specifically, the recording method is a method in which a dimensional error held by each of the L0 layer and the L1 layer, as shown in FIG. 4(a), and a pasting error in the L0 layer and the L1 layer, as shown in FIG. 4(b), are added; namely, in which a so-called margin is considered.
Patent document 1: Japanese Patent Application Laid Open NO. 2000-311346
Patent document 2: Japanese Patent Application Laid Open NO. 2001-23237